1. Technical Field of the Invention
This invention relates to an actuator for a valve to be loaded in a rotary valve, such as a ball valve or a butterfly valve, and more particularly to an actuator so constructed as to use an electric motor as a power source and effect rotary operation of the valve shaft of a rotary valve, such as a ball valve or a butterfly valve, through a speed-reducing mechanism.
2. Description of the Related Art
The conventional actuator for a valve is furnished with a speed-reducing mechanism having a train of spur gears disposed as meshed and connected in a multiplicity of stages between an output shaft and the rotary shaft of a motor with a view to reducing the turning force of the motor and transmitting the reduced force to the output shaft and, by resorting to this medium of the speed-reducing mechanism, enabled to reduce the turning force from the motor and transmit the reduced force to the output shaft and then to the valve shaft connected to the output shaft and accomplish a switching operation of the ball or disc as the valve body. Since the actuator for the valve which has the aforementioned built-in speed-reducing mechanism requires to secure a wide space between the output shaft and the rotary shaft of the motor because of the construction having numerous spur gears disposed as meshed and connected in numerous stages, it entails such problems as incurring a large addition to the size of actuator, complicating the construction, increasing the number of component parts and boosting the cost.
As a solution to these problems, JP-A 2002-115748 has been proposed. JP-A 2002-115748 discloses an actuator having a built-in speed-reducing mechanism that is capable of acquiring a large reduction ratio with a small number of toothed wheels.
The speed-reducing mechanism incorporates therein a speed-reducing mechanism and comprises an eccentric body disposed coaxially around an output shaft serving to transmit rotation to the exterior of the housing, enabled by a motor to rotate relative to the output shaft and possessed of an outer peripheral circle eccentric relative to the rotary shaft, an annular inner sun gear adapted to have the outer and inner peripheries thereof disposed coaxially with the output shaft, furnished along the inner peripheral surface thereof with an internal gear and enabled to rotate around the output shaft relative to the housing, an external planet gear possessed of an internally bored circle coaxial with the outer peripheral circle of the eccentric body and an external gear disposed on the outer periphery, disposed as extrapolated rotatably relative to the eccentric body and enabled by having the outer gear thereof partially meshed with the annular inner sun gear to produce a revolving motion along the inner periphery of the inner sun gear, a rotation-transmitting mechanism for transmitting the rotation of the external planet gear to the output shaft, and a rotation-interrupting mechanism for restraining and releasing the rotation of the annular sun gear relative to the housing. It allows the actuator to be miniaturized.
The actuator incorporating therein the conventional speed-reducing mechanism which is capable of acquiring a large reduction ratio with a small number of toothed wheels (as disclosed in JP-A 2002-115748, for example) lacks a mechanical means to control the rotation of the output shaft in the prescribed range of angle when the turning force from the motor is transmitted to the output shaft through the speed-reducing mechanism. In the ON or OFF operation effected by depressing the point of contact of a limit switch by means of a cam, for example, when the relation of relative positions of the cam and the limit switch produces a deviation or when the limit switch produces a malfunction, the actuator possibly fails to switch the valve body within the prescribed range of valve opening and induces erroneous supply of a fluid. The actuator entails the same problem when it is manually operated during a power failure.
Incidentally, the rotation-interrupting mechanism of this conventional operating principle is aimed at restraining and releasing the rotation of an annual inner sun gear relative to the housing and is not intended to control the range of angle of rotation of the output shaft during the motor operation or the manual operation.
The actuator incorporating therein the conventional speed-reducing mechanism which is capable of acquiring a large reduction ratio with a small number of toothed wheels (as disclosed in JP-A 2002-115748, for example), in changing the turning force from the motor, namely the magnitude of the output torque, encounters such problems in terms of operation and economy as requiring to change the diameter of the output shaft in conformity with the magnitude of the output torque and, at the same time, replace the aperture-detecting members of the valve body, such as cam members and limit switches and other component parts mounted on the actuator, or vary the positions of their disposition, and prepare the parts and the component parts mentioned above and fitting plates for fixing and disposing these parts and component parts in the actuator for each output torque and assemble them.
Further, when the actuator in the process of replacing the aperture-detecting members of the valve body, such as limit switches and other mounted parts, or altering the positions of disposition thereof produces a deviation in the sites of attachment or the positions of disposition, it encounters the problem of giving rise to a minute deviation in the relation of relative positions of the cam member and the limit switch particularly during the ON or OFF operation effected by depressing the point of contact of the limit switch by means of the cam member attached to the output shaft and disabling the switching position of the value from being accurately detected.
The conventional actuator, for the sake of disposing the cam member as fixed to the outer peripheral site of the output shaft through a fitting member, such as screws, requires the diameter of the output shaft and the diameter of the hole in the cam member to be finished with high accuracy and, for fear that the tightening of the fixing member, such as screws, during the attachment of the cam member will produce a deviation in the fixing position of the cam member or induce the output shaft to yield to plastic deformation, requires the fixing work to use highly advanced adjusting and fixing techniques.
When the actuator, in consequence of the continued use thereof, suffers the fixing position of the cam member to deviate particularly under the influence of the vibration peculiar to the electric motor, the ON or OFF operation effected by depressing the point of contact of the limit switch encounters such problems as giving rise to deviation in the relation of relative positions of the cam member and the limit switch, preventing the switching position of the valve from being accurately detected and forcing the actuator to develop a fault.
The occurrence of an excessive torque constitutes one main cause for the fault of the actuator. When an extraneous substance is caught in the train of gears forming a motive power transmitting mechanism in the course of an electric operation (conducting period), the gears which are disposed as fixed in a manner incapable of generating a relative rotation with the motive power transmitting shaft by key coupling or pin coupling result in developing an unduly large load by biting the extraneous substance and possibly suffering the electric motor to incur a trouble, such as burning.
Similarly during the course of a manual operation (period of power failure), when an extraneous substance is caught in the train of gears forming the motive power transmitting mechanism, the output torque generated in excess of the prescribed range, namely the excess torque, by the manual operation gives rise to such problems as exerting an unduly large load on the gears forming the gear unit, eventually inflicting damage on the gear unit and preventing the motive power transmission by the manual operation from being effected and the switching operation of the valve body from being executed.
Further, since the actuator disclosed in JP-A 2002-115748 has the manually operated shaft disposed coaxially with the output shaft, it requires the whole of itself including the manually operated shaft to change their sizes whenever the diameter of the output shaft is varied by the diameter of the valve.
Moreover, for the purpose of enabling the torque to be effectively transmitted from the manually operated shaft to the valve body, the manually operated shaft is required to have a diameter large enough to withstand the torque. When the position-detecting device is adapted to be disposed on the shaft of this large diameter, it will possibly result in adding to the whole size of the actuator.
The speed-reducing mechanism of this actuator incurs an addition to the number of component parts because it is provided with the rotation-interrupting mechanism. This rotation-interrupting mechanism retains the inner sun gear as fixed through the engagement between the clutch gear and a rotation-stopping groove. Thus, these mechanisms entail the problem of durability because the torque load is concentrated thereon during the normal electric operation of the actuator.
When this actuator is provided with a valve opening-indicator, the valve opening-indicator is fated structurally to be disposed coaxially with the manually operated axis. During the course of the manual operation, therefore, the valve opening-indicator will possibly approximate closely to the manual operation part and become difficult to read. Further, since the ordinary valve opening-indicator is adapted to be covered as by a clear acrylic sheet which is flush with the upper surface or lateral surface of the actuator and consequently enabled to show the indicating part thereof on the actuator interior side through this acrylic sheet to the observer outside, it generates directionality during the visual observation and becomes difficult to see unless it is looked at in a specific direction and renders the manual operation inconvenient.
When the accuracy of rotation of the output shaft is warped by the backlash of the gears, this malfunction results in affecting the indication of the valve opening-indicator and rendering the accurate indication of the valve opening difficult.
Further, in the conventional actuator that is furnished with a built-in planet gear mechanism (as disclosed in JP-A 2002-115748, for example), the outer gear is meshed in an inscribing manner with the inner gear so that the rotating output to the outer shaft may be derived from the vibrating motion of the outer gear. When the carrier that is an eccentric rotating body is rotated to push the annular inner sun gear, the radial load originating in the vibrating motion is exerted on the anchoring part of the output shaft, and the output shaft is structurally exposed to the force vibrating perpendicularly to the direction of the axis thereof.
When the output shaft is anchored with a bush that is liable to impart a gap to the anchoring part, therefore, the output shaft possibly incurs the phenomenon of emitting vibration, namely the so-called shaft run-out.
When the output shaft produces a shaft run-out, the plate, for example, that is disposed on the extended part of the output shaft is made to vibrate. With this plate, it becomes difficult to effect accurately the ON or OFF operation of the control switch. The run-out of the output shaft brings such problems as rendering accurate detection of the state of rotation of the output shaft difficult, suffering the output shaft to develop a malfunction and preventing the valve body from being accurately controlled in the state of opening, closing and a prescribed intermediate opening during the course of switching the valve.
This invention, which has been developed with a view to solving the conventional problems, is aimed at providing an actuator for a valve to be mounted in a rotary valve, such as a ball valve or a butterfly valve, and more particularly an actuator so constructed as to effect a rotating operation of the valve shaft of a ball valve or a butterfly valve through an inner planet gear speed reducing mechanism which uses an electric motor, for example, as the source of motive power and is capable of obtaining a large reduction ratio with a small number of gears, thereby making it possible to control the angle of the valve accurately, detect the open-close position of the valve accurately and materialize reduction of size and weight and saving of cost owing to the simplification of structure. This invention is aimed further at providing a compact and economically excellent actuator for a valve which enables valve opening-detecting members, such as limit switches and potentiometers and other component members to be put to common use without reference to the size of the output torque and ensures accurate detection of the open-close position of the valve.